Magnesium alloy and heat treatment method thereof

ABSTRACT

A magnesium alloy having excellent castability and creep properties has compositions of 1.0-6.0 wt % Zn, 0.5-3.0 wt % Ca, 1.0 wt % or less Zr, 1.0-5.0 wt % at least one lanthanoid, the remainder being Mg and unavoidable impurities. This magnesium alloy undergoes heat treatment of heating the magnesium alloy to 430-470 ° C., quenching, and then heating to 150-250 ° C. for aging. Hot tearing and temperature strength are improved by the addition of an element which is effective for causing eutectic reaction and peritectic reaction with Mg and making Mg particles divied finely.

1. FIELD OF THE INVENTION

[0001] The present invention relates to a magnesium (Mg) alloy habing excellent castability for die casting and creep characteristics, which contains Zn, Ca, Zr and at least one rare earth element in predetermined ratios, as the essential components, and a heat treatment method for further improving mechanical properties (particularly hardness, creep strength and fatigue strength) of a magnesium alloy.

2. DESCRIPTION OF THE RELATED ART

[0002] In recent years, needs for a light-weight material are increasing. In paticular, a magnesium alloy, the smallest density of practical alloys is noted as materials for aircrafts and automobiles.

[0003] However, due to poor high-temperature strength of this magnesium alloy or in order to increase heat resistance thereof, expensive elements such as yttrium (Y) or silver (Ag) are added thereto. As a result, such an alloy becomes a very expensive alloy and also hot teating is liable to occur where it is used for high pressure die casting. Thus, the magnesium alloy is used only for very limited purposes.

[0004] So, various magnesium alloys are proposed to solve the above-mentioned problems. For examples, Japanese Patent Application Laid-open No. Hei (hereinafter referred to as JP-A-) 7-18364 descloses a Mg—Zn—Ca alloy haing improed heat resistance and creep characteristics by adding relatively inexpensive calcium (Ca) in place of expensive elements such as yttrium.

[0005] Further, in the similar alloy system, JP-A-6-25791 discloses a Mg—Zn—Ca—Cu alloy having improved room-temperature strength and high temperature strength by adding copper (Cu) in addition to calcium. For example, the above-mentioned JP-A-6-25791 discloses a magnesium alloy having excellent room-tempera ture and high temperature strength, which comprises 3-8 wt % of zinc, 0.8-5 wt % of calcium, and 0-10 wt % of copper, and if necessary, further comprising 2 wt % or less of each of manganese, zirconium and silicon, and 4 wt % or less of at least one of rare earth element, the remainder being magnesium and unavoidable impurities. The unit of wt % hereinafter means % by weight.

[0006] Similarly, as a magnesium alloy having excellent room-temperature and high temperature strength, JP-A-6-200348 discloses a magnesium alloy comprising (a) 0.5-5 wt % of lanthanoids, (b) 0.5-5 wt % of calcium, (c) 1.5 wt % or less of manganese and/or 1.5 wt % or less of zirconium, (d) if required, at least one member selected from the group consisting of 1-9.5 wt % of aluminum, 1-7.5 wt % of zinc and 0.5-4 wt % of silver, (e) if further required, at least one element selected from the group consisting of 5.5 wt % or less of yttrium, 1.5 wt % or less of strontium and 10 wt % or less of scandium, the remainder being magnesium and unavoidable impurities.

[0007] In general, the addition of Ca decreases elongation of a magnesium alloy. Therefore, the alloys were subjected to solution heat treatment in JP-A-6-25791 and 6-200348. Further, general Mg—Zn based aloys such as ZE41 increase its strength only by artificially aging at relatively low temperature (T5 treatment). This is due to that this kind of alloy compound can be formed at relatively low temperature. On the other hand, usual Mg-R.E. based alloys such as EZ33 undergo T5 treatment because it is difficult to complete a solution heat treatment to them conducted.

[0008] However, sufficient high temperature strength cannot be imparted to general magnesium alloys which do not contain Ca, such as Ze41 or EZ33. On the other hand, heat treatment to the magnesium alloys was not sufficiently investigated in JP-A-6-25791 and 6-200348, and failed making such alloys sufficiently exhibit their characteristics.

[0009] And, those alloys have the disadvantages that hot tearing tends to occur in die casting, and therefore it is difficult to conduct die casting. Further, creep properties of those alloys are good as compared with other magnesium alloys, but are poor as compared with the conventional alloys having added thereto expensive elements such as yttrium. Therefore, a magnesium alloy which is cheap and excellent both in castability for die casting and high temperature creep properties have not been known yet.

SUMMARY OF THE INVENTION

[0010] As a result of extensive investigations and various systematic experiments to overcome the above-mentioned problems involved in the related arts, the present inventors have reached the present invention.

[0011] Accordingly, an object of the present inention is to provide a magnesium alloy having excellent castability for high pressure die casting and high temperature creep properties.

[0012] Another object of the present invention is to provide a heat treatment method of a magnesium alloy by applying a casting method other than a high pressure die casting for further improving mechanical properties, such as hardness, creep strength and fatigue strength, of the conventional Mg—Ca based alloys such as a Mg—Zn—Ca based allys or Mg-R.E. (lanthanoids)-Ca based alloy.

[0013] According to a first aspect of the present invention, there is provided a magnesium alloy having excellent high pressure die casting properties and creep properties, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less (exclusive of 0 wt %); at least one rare earth element in the content range of 1.5 to 2.7 wt %; the remainder being magnesium; and unavoidable impurities.

[0014] According to a second aspect of the present invention, there is provided a magnesium alloy having excellent high pressure die casting properties and creep characteristics, comprising: Calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less; at least one rare earth element in the content range of 1.5 to 2.7 wt %; alminum (Al) in the content of 2.0% or less (exclusive of 0 wt %); manganese (Mn) in the content of 1.0 wt % or less; the remainder being magnesium; and unavoidable impurities.

[0015] According to a third aspect of the present inventiion, there is provided a heat treatment method of a magnesium alloy, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less (exclusive of 0 wt %); at least one rare earth element in the content range of 1.5 to 5.0 wt %; the remainder being magnesium; and unavoidable impurities, comprising the steps of: a) heating the magnesium alloy in the temperature range of 430 to 470° C.; b) quenching the heated aloloy; and c) heating the alloy to in the temperature range of 150 to 250° C. to age the same.

[0016] The present inventors have noted the folowing points for the above-mentioned problems in the related arts.

[0017] Almost all of magnesium products are high pressure die casting product. However, the above-mentioned Mg—Zn—Ca alloys in the related arts are liable to cause hot tear, and it is difficult to apply the alloys to high pressure die csting. Therefore, the present inventors have intended to improve castability of these alloys and to increase high temperature characteristics of these alloys as high as those of an aluminum alloy and the conventional magnesium alloys which have expensive elements such as yttrium as an additive.

[0018] As a result of various investigations to solve the above problem, the present inventors have found that casting crack can be suppressed if an element which causes eutectic reaction with Mg and an element which has an effect for finely dividing Mg particles and causes peritectic reaction with Mg are added in combination, to the Mg—Zn—Ca alloy. Through further investigations on these additives, the inventors have come to know that rare earth elements which have a relatively low solubility (Lanthanoids) to Mg, e.g. lanthanum (La), cerium (Ce), praseodymium (Pr) neodymium (Nd) and misch metal, a mixture of those elements are effective in eutectic reaction. They also discovered that zirconium (Zr) is very effective in causing peritectic reaction, which finely divides and spheroidize crystal particles.

[0019] Mechanism that the magnesium alloy of the present invention has excellent castability and creep characteristics together is not yet clarified, but it is considered as follows.

[0020] The magnesium alloy of the present invention improves formability of hot tear and high temperature strength of the conventional Mg—Zn—Ca alloy. The decrease of formability of hot tear is considered to be due to that an appropriate increase of eutectic compound, narrowed solidification temperature, and finely dividing a —Mg particles are acted cooperatively. The increase of machaical properties is speculated to be caused by fine a —Mg particles solid solution hardening and precipitation hardening.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a plane view of a test piece which was high pressure die cast.

[0022]FIG. 2 is a graph showing a creep curve of a high pressure die casting material.

[0023]FIG. 3 is a graph showing high temperature strength of a casting material.

[0024]FIGS. 4A and 4B are microphotographs of a gravity casting metal structure when a magnesium alloy is treated under solution heat treatment conditions at 41° C. for 24 hours, and then quenched in hot water at about 80° C.

[0025]FIG. 4A has a magnification of 500 times, and

[0026]FIG. 4B has a magnification of 1,000 times.

[0027]FIGS. 5A and 5B are microphotographs of a gravity casting metal structure when a magnesium alloy is treated under solution heat treatment conditions at 440° C. for 24 hours, and then quenched in hot water at about 80° C.

[0028]FIG. 5A has a magnification of 500 times, and

[0029]FIG. 5B has a magnification of 1,000 times.

[0030]FIGS. 6A and 6B are microphotographs of a gravity casting metal structure when a magnesium alloy is treated under solution heat treatment conditions at 465° C. for 24 hours, and then quenched in hot water at about 80° C.

[0031]FIG. 6A has a magnification of 500 times, and FIG. 5B has a magnification of 1,000 times.

[0032]FIGS. 7A and 7B are microphotographs of a gravity casting metal structure when a magnesium alloy is treated under solution heat treatment conditions at 480° C. for 24 hours, and then quenched in hot water at about 80° C.

[0033]FIG. 7A has a magnification of 500 times, and

[0034]FIG. 7B has a magnification of 1,000 times.

[0035]FIG. 8 is a graph showing change in hardness of an alloy when aging temperature and aging time are changed.

[0036]FIG. 9 is a graph showing a tensile strength of an alloy which was subjected to heat treatment and an alloy was merely obtained by casting without heat treatment (gravity cast).

[0037]FIG. 10 is a graph showing time that an alloy merely obtained by casting without heat treatment requires to reach 0.1% creep elongation under conditions of 150° C. and 100 MPa (gravity cast).

[0038]FIG. 11 is a graph showing a rotary bending fastigue test result from an alloy which was subjected to heat treatment and an alloy which was merely obtained by casting without heat treatment (gravity cast).

[0039]FIG. 12 is a graph showing hardness of an alloy which was subjected to heat treatment, and an alloy which was merely obtained by casting without heat tretament (gravity cast).

DETAILED DESCRIPTION OF THE INVENTION

[0040] Zn in the magnesium alloy of the present invention improves room temperature strength and castability of the alloy. If the Zn content is less than 1% by weight with respect to a total alloy weight which is hereinafter refered to as % by weight or wt %, the effect thereof is insufficient, and hot tear is liable to occur. On the other hand, if the Zn content exceeds 6% by weight, creep properties are deteriorated. Therefore, the amount of Zn to be added is 1.0 to 6.0% by weight, and preferably 2 to 4% by weight so that the room temperature strength is improved, and creep properties do not substantially deteriorate.

[0041] Ca improves static strength of Mg and creep properties of the alloy. If the Ca content is less than 0.5% by weight, the alloy cannot sufficiently reinforced. On the other hand, if the Ca content exceeds 3.0% by weight, elongation is decreased and also many hot tearing occur in high pressure die casting. Therefore, the amount of Ca to be added is 0.5 to 3.0% by weight. Since improvement of creep characteristics due to addition of Ca almost saturates in an amount of about 1.0% by weight, the amount of Ca added is preferably 0.7 to 1.5% by weight.

[0042] Zirconium must be added in an amount of more than 0% by weight so as to exhibit the finely dividing effect. The Zr content is preferably 0.4% by weight or more, and more preferably 0.5% by weight or more. However, if the Zr content exceeds 1% by weight, a melting poingt of the alloy becomes higher, so that zirconium does not disperse uniformly, which is industrially unfavorable. Therefore, the Zr content is generally 1.0% or less by weight, preferably 0.4 to 1.0% by weight, and more preferably 0.5 to 1.0% by weight.

[0043] The rare earth element that forms an eutectic compound improves the high pressure die casting properties of the alloy and also increases strength thereof. In particular, it is effective to use at least one of rare earth elements having a relatively low solubility (lanthanoids) in magnesium and showing a eutectic type constitutional diagram at magnesium side, such as lanthanum, cerium, praseodymium or neodymium. If the total content of the rare earth element is less than 1.5% by weight, an effect for suppressing hot tear is small, and on the other hand, if the total content thereof exceeds 5% by weight, the alloy becomes brittle due to increase of the eutectic compound. Therefore, the rare earth element content is 1.5 to 5% by weight, and preferably 2.0 to 2.7% by weight for sufficiently exhibiting characteristics of the rare earth element.

[0044] The magnesium alloy of the present invention may contain aluminum in an amount of 2.0% by weight or less in order to improve a static strength. Incidentally, if the proportion of aluminum exceeds 2.0% by weight, creep properties may deteriorate. The magnesium alloy of the present invention may also contain manganese in an amount of 1.0% by weight or less for the purpose of improving corrosion resistance and creep characteristics.

[0045] One example of a process for producing the magnesium alloy of the present invention is briefly explained below.

[0046] That is, the alloy of the present invention is produced by adding each component element other than magnesium to molten magnesium in the form of pure metal, alloy, chloride or fluoride, or by combining various mother alloys containing magnesium or each component element.

[0047] It should be noted that regarding zirconium, it is necessary to stir a melt, considering sustained time of a finely dividing effect. During melting work, as well as conventional magnesium, it is preferred to conduct, if required and necessary, anti-combustion or refining with SF₆, flux or the like. By die casting this melt, a material having a fine structure and good mechanical characteristics can be obtained.

[0048] The magnesium alloy of the present invention (hereinafter simply referred to as “the alloy”) is investigated in detail on structure change to temperature. As a result, it has been found that re-melting temperature of the alloy is about 60° C. higher than that of the conventional Mg—Zn—Ca based alloy. Therefore, utilizing this fact, invenstigations have been made, regarding a material which can be heat-treated such as a gravity casting material, to further increase strength of the material by heat treatment.

[0049] In order to improve high temperature creep strength, it is advantageous to precipitate a compound after dissolving many solutes in a matrix. Therefore, the alloy is subjected to a solution heat treatment. The temperature of the solution heat treatment is 430° C. or more to 470° C. or less. If the temperature is less than 430° C., almost all of compounds maintain its form, and a solubility of element in the matrix does not almost change. On the other hand, if the temperature exceeds 470° C., a low temperature melting portion re-melts, and defects are formed. Therefore, the solution heat treatment temperature must be 430 to 470° C. Further, in order to sufficiently dissolve the compounds in the matrix, 5 hours or more are necessary, and the solubility substantially reaches the dissolution limit in 24 hours. Therefore, it is sufficient for the solution heat treatment time to be 5 to 24 hours. After this treatment, the alloy is quenched in an appropriate medium (such as hot water or forced air cooling) so as not to produce coarse compounds in the course of cooling.

[0050] The alloy thus subjected to the solution heat treatment is aged by heating it to 150 to 250° C. (compounds are precipitated, thereby hardening the alloy). If the aging temperature is less than 150° C., it takes long time for hardening. On the other hand, if the aging temperature exceeds 250° C., precipitation hardening does not occur remarkably. Therefore, the aging temperature is 150 to 250° C., and preferably 150 to 200° C. An aging time varies depending on the temperature, but it sufficiently hardens for 0.5 to 24 hours. The aging conditions are preferably 180 to 200° C. and 0.5 to 2 hours, in view of temperature control and productivity.

[0051] In the alloy, Ca finely divides precipitates to improve heat resistance of the alloy. In other words, Ca suppresses growth rate of precipitates. This effect is further remarkable by adding Ca in an amount of 0.5% by weight or more. The effect by addition of Ca saturates at the maximum solubility of Ca. Therefore, even if Ca may be added in an amount of about 1% by weight or more, there is no great change. Further, toughness of the alloy decreases as the amount of Ca added increases. Therefore, addition of Ca in an amount exceeding 3.0% by weight is not preferable. Therefore, Ca is added in an amount of 0.5 to 3.0% by weight. Considering toughness of the alloy to be obained, it is preferable to add Ca in an amount of 0.5 to 1% by weight.

[0052] In the alloy, Zn is related to solubility strengthening, precipitation strenghthening and castability. In order to strengthen the alloy with Zn, it is neessary to add Zn in an amount of 1% by weight or more. However, if Zn is added in an amount exceeding 6% by weight, a large amount of compounds precipitates in the alloy, resulting in decrease of toughness of the alloy. Therefore, Zn is added in an amount of 1.0 to 6.0% by weight.

[0053] In the alloy, Zr is an element for finely dividing crystal particles. However, even if Zr is added in an amount exceeding 1% by weight, it is difficult for Zr to dissolve in the alloy. Therefore, Zr is added in an amount of 0 to 1.0% by weight. In other words, the alloy may not contain Zr. When the alloy contains Zr, the amount of Zr added is 1% by weight or less.

[0054] Rare earth element (R.E. for short) including lanthanoid improves heat resistance and castability of the alloy. To exhibit these effect remarkably, it is necessary to add rare earth element in an amount of 1% by weight or more. If rare earth element to be added exceeds 5% by weight, not only the effect of improving the castability is saturated, but also reaction between elements and a large amount of compounds decreases the toughness of the alloy. Therefore, rare earth element to be added is preferably 1.0 to 5.0% by weight.

[0055] The present inventiion is described in more detail by reference to the following Examples and Comparative Examples. Test Examples in the Examples relating to the process of the present invention include the Examples and Comparative Examples, but are consideration examples of heat treatment conditions. Therefore, those are considered as the test examples and shown in the consecutive number.

[0056] A. Alloy of the Present Invention

EXAMPLES 1 TO 3

[0057] Inner surface of high chromium alloy steel (Japanese Industrial Standard SUS430) crucible preheated in an electric furnace was coated with magnesium chloride flux. Pure magnesium was introduced into the crucible and melted. Metalic calcium-zinc-cerium based misch metal (Mm) was added to the resulting melt maintained at 700° C. The temperature of the melt was further raised to 780° C., an Mg-Zr alloy was added thereto, and the melt was stirred. After sufficient stirring, it was confirmed that those metals were completely dissolved, and flux refining was then conducted. After completion of the refining, the temperature was maintained at 780° C. Incidentally, during the melting work, a mixed gas of carbon dioxide gas and SF₆ was blown to the melt surface at a flow rate of 0.2 liter/minute for preventing combustion, and also flux was appropriately strewed on the melt surface. The alloy melt thus obtained was die cast into a test piece shape as shown in FIG. 1. Occurrence of hot tearing in high pressure die casting was confirmed visually or with × ray flaw detection, and sensibility of formability of hot tear of the alloy was qualitatively evaluated by ∘ and ×. The results obtained are shown in Table 1 below.

COMPARATIVE EXAMPLES 1 TO 5 AND AZ91D

[0058] By using each component in the proportion shown in Table 1, each alloy test piece of Comparative Examples 1 to 5 was produced in the same manner as in the above Examples, and sensibility of hot tear thereof was evaluated in the same manner as in the above Examples. Further, a test piece was similarly produced using AZ91D which was a practical magnesium alloy, and its sensibility of hot tear thereof was evaluated in the same manner as above. The results obtained are shown in Talbe 1 below. TABLE 1 Ce + La + Al Zn Nd + Pr Ca Zr hot Alloy (wt %) (wt %) (wt %) (wt %) (wt %) tearing AZ91D 8.5 0.9 — — — ◯ Comparative — — — 0.71 — X Example 1 Comparative — 2.0 1.95 0.7  — Example 2 Comparative — 2.0 — 0.8  0.6 X Example 3 Comparative — — 1.9  0.75 0.7 X Example 4 Comparative — 2.0 1.1  0.8  0.7 X Example 5 Example 1 — 2.0 1.85 0.78 0.6 ◯ Example 2 — 4.0 1.55 0.76 0.7 ◯ Example 3 — 2.0 2.57 0.30 0.8 ◯

[0059] It is apparent from the results shown in Table 1 aboe that the magnesium alloys of Examples 1 to 3 have formability of hot tearing equal to that of AZ91D which is a practical magnesium alloy. In contrast to this, the magnessium alloys of Comparative Examples 1 to 5 were apt to generate hot tearing. In more detail, the magnesium alloys of Comparative Examples 1-5 are alloys which do not contain at least one of the essential components (except for magnesium) in the alloy of the present invention (alloys of Comparative Examples 1 to 4) or the alloy wherein the rare earth element content is outside the range of the present invention (magnesium alloy of Comparative Example 5), and the formability of hot tearing easily occured in each alloy. Thus, the essential components other than magnesium are effective in the magnesium alloy of the present invention. In particular, even if the rare element content is slightly away from the range of the present invention, the hot tearing was apt to generate. Therefore, the effectiveness of the rare earth element content in the present invention can be understood from this fact.

[0060] Creep properties and mechanical properties of various magnesium alloys thus produced are shown in FIGS. 2 and 3.

[0061]FIG. 2 shows creep curves of material (test piece of FIG. 1) produced from various magnesium alloys. The curve shows a change with the passage of time (horizontal axis: Ms) of strain (vertical axis). In FIG. 2, AZ91, AS41 and AE42 which are the conventional magnesium alloy each show large strain rate under the conditions of 150° C. and 50 MPa, and the strain rate increases with the passage of time. On the other hand, an Mg-2% Zn-1%Ca alloy shows creep properties similar to those of AE42 under the conditions of 150° C. and 64 MPa. However, the alloy of the present invention (Mg-2% Zn-0.6%Ca-2% Mm-0.5% Zr, wherein Mm is misch metal) shows very low strain rate similar to QE22 gravity cast material under the conditions of 150° C. and 64 MPa, and the strain rate does not substantially increase with the passage of time.

[0062]FIG. 3 shows change of σ_(B/ρ) (vertical axis: tensile strength/density) of die cast materials (test piece of FIG. 1) produced from various magnesium alloys, by the test temperature (horizontal axis: K). It is apparent that the alloy of the present invention is equal or superior at a high temperature to QE22-T6, ZE41-T5 and AC8A-T6 produced by gravity die casting (wherein T5 and T6 show heat treatment) which are the conventiional heat treated alloys, and also is superior to AZ91 produced by high pressure die casting.

[0063] B. Method of the Present Invention

EXAMPLE 4

[0064] I. Production of Magnesium Alloy

[0065] A melt of magnesium alloy was prepared in the same manner as in Examples 1 to 3, and a boat form mold was cast from the melt. The compositions of this alloy were Mg-2% Zn-2% Mm-0.6%Ca-0.6% Zr (% is % by weight).

[0066] II. Investigation on Solution Heat Treatment Temperature

[0067]FIGS. 4A to 7B each show a microphotograph of a metal structure when the magnesium alloy obtained above was maintained in argon stream at a high temperature, and then quenched in hot water at about 80° C. (solution heat treatment). FIGS. 4A and 4B are microphotographs where the solution heat treatment temperature is 410° C. and the solution heat treatment time is 24 hours, in which FIG. 4A is a microphotograph having a magnification of 500 times, and FIG. 4B is a microphotograph having a magnification of 1,000 times (Test Example 1). FIGS. 5A and 5B are microphotographs where the solution heat treatment temperature is 440° C. and the solution heat treatment time is 24 hours, in which FIG. 5A is a microphotograph having a magnification of 500 times, and FIG. 5B is a microphotograph having a magnification of 1,000 times (Test Example 2). FIGS. 6A and 6B are microphotographs where the solution heat treatment temperature is 465° C. and the solution heat treatment time is 24 hours, in which FIG. 6A is a microphotograph having a magnification of 500 times, and FIG. 6B is a microphotograph having a magnification of 1,000 times (Test Example 3). FIGS. 7A and 7B are microphotographs where the solution heat treatment temperature is 480° C. and the solution heat treatment time is 24 hours, in which FIG. 7A is a microphotograph having a magnification of 500 times, and FIG. 7B is a microphotograph having a magnificatiion of 1,000 times (Test Example 4).

[0068] There is no great change on the metal structure of the alloy until the solution heat treatment temperature of 410° C. (Test Example 1), but black compound in the metal structure of the alloy decreases at the solution heat treatment temperature of 440° C. (Test Example 2) and the solution heat treatment temperature of 465° C. (Test Example 3). This fact shows that the black compound decomposes, and the element is dissolved in the matrix phase. However, the low temperature melting portion initiates remelting at the solution heat treatment temperature of 480° C. (Test Example 4), and defects are formed in the alloy.

[0069] III. Investigation on Aging Teperature

[0070]FIG. 8 is a graph (age hardening curve) showing change in hardness (Vickers hardness: HV) of the alloy where the aging temperature and the aging time are changed, in aging the alloy of Test Example 3 (solution heat treatment at 465° C. for 24 hours). Remarkable hardening is not recognized at the aging temperature of 100%C (Test Example 5). Hardness is increased with the passage of time at the aging temperature of 150° C. (Test Example 6). Further, peak hardness is shown in a short time at the aging temperature of 200° C. (Test Example 7). It is therefore understood that age hardening occurs in a short time at the aging temperature of 200° C. or more. On the other hand, in the alloy (Test Example 8) which is merely maintained at 465° C. from room temperature after casting, change in hardness is not substantially observed.

[0071] IV. Investigation on Influence of the Presence or Absence of Quenching and Aging to Tensile Strength

[0072]FIG. 9 shows a tensile strength of an alloy (Test Example 9) obtained by quenching the alloy of Test Example 3 in hot water and then aging the same at 200° C. for 2 hours, and that of an alloy (Test Example 10) obtained by merely casting without heat treatment. It is apparent from FIG. 9 that the alloy of Test Example 9 with heat treatment increases its tensile strength as compared with that of the alloy of Test Example 10.

[0073]FIG. 10 is a graph showing time until reaching 0.1% of creep strain under the conditions of 150° C. and 100 MPa in the alloys of Test Example 9 and Test Example 10. It is apparent from FIG. 10 that the alloy of Test Example 9 with heat treatment took longer time than that of the alloy of Test Example 10 until reaching 0.1% of creep strain.

[0074]FIG. 11 is a graph showing the results of rotary bending fatigue test of the alloys of Test Examaple 9 and Test Example 10. It is apparent from FIG. 11 that the alloy of Test Example 9 improved 10 times in fatigue limit as compared with the alloy of Test Example 10.

EXAMPLE 5

[0075] I. Production of Magnesium Alloy

[0076] A magnesium alloy was produced in the same manner as in Example 4 except that the Mg-Zr alloy was not added. This alloy had a composition of Mg-2% Zn-2% Mm-0.7%Ca (% is % by weight).

[0077] II. Investigation on Influence of the Presence or Absence of Heat Treatment to Hardness of Alloy

[0078] The magnesium alloy obtained above was heated at 465° C. for 24 hours, quenched in hot water, and then aged at 200° C. for 2 hours. Vickers hardness (Hv) of the alloy thus treated was then measured.

[0079] For the sake of comparison, a magnesium alloy obtained by merely casting without the above heat treatment was measured with Vickers hardness (Hv).

[0080] The results obtained are shown in FIG. 9. It is apparent from FIG. 9 that the hardness of the magnesium alloy is improved by the heat treatment.

[0081] The magnesium alloy according to a first aspect of the present invention has excellent high pressure die casting properties, very high strength at a high temperature, and also excellent creep properties. Further, the magnesium alloy according to the first aspect of the present invention can use inexpensive rare earth elements as a substitute for expensive elements such as yttrium in the rare earth elements, and therefore can be produced relatively inexpensively.

[0082] The magnesium alloy according to a second aspect of the present invention has, in addition to the effect in the magnesium alloy according to the first aspect of the present invention, improved static strength and corrosion resistance, and also further improved creep properties.

[0083] If the heat treatment method of a magnesium alloy according to a third aspect of the present invention is used, precipitates which are stable at a high temperature are finely dispersed in α-Mg in the magnesium alloy to be obtained. Therefore, the mechanical properties of such a magnesium alloy are improved as compared with the magnesium alloys obtained by the conventional method, and in particular, creep strength and fatigue strength are improved. 

What is claimed is:
 1. A magnesium alloy having excellent high pressure die casting properties and creep characteristics, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less (exclusive of 0 wt %); at least one rare earth element in the content range of 1.5 to 2.7 wt %; the remainder being magnesium; and unavoidable impurities.
 2. A magnesium alloy according to claim 1, wherein the content range of Zn is 2.0 to 4.0 wt %.
 3. A magnesium alloy according to claim 1, wherein the content range of Ca is 0.7 to 1.5 wt %.
 4. A magnesium alloy according to claim 1, wherein the content range of Zr is 0.4 to 1.0 wt %.
 5. A magnesium alloy according to claim 4, wherein the content range of Zr is 0.5 to 1.0 wt %.
 6. A magnesium alloy according to claim 1, wherein the content range of the rare earth element is 2.0 to 2.7 wt %.
 7. A magnesium alloy according to claim 1, wherein the rare earth element is a lanthanoid.
 8. A magnesium alloy having excellent high pressure die casting properties and creep characteristics, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less; at least one rare earth element in the content range of 1.5 to 2.7 wt %; alminum (Al) in the content of 2.0% or less (exclusive of 0 wt %); manganese (Mn) in the content of 1.0 wt % or less; the remainder being magnesium; and unavoidable impurities.
 9. A magnesium alloy according to claim 8, wherein the content range of Zn is 2.0 to 4.0 wt %.
 10. A magnesium alloy according to claim 8, wherein the content range of Ca is 0.7 to 1.5 wt %.
 11. A magnesium alloy according to claim 8, wherein the content range of Zr is 0.4 to 1.0 wt %.
 12. A magnesium alloy according to claim 11, wherein the content range of Zr is 0.5 to 1.0 wt %.
 13. A magnesium alloy according to claim 8, wherein the content range of rare earth element is 2.0 to 2.7 wt %.
 14. A magnesium alloy according to claim 8, wherein the rare earth element is a lanthanoid.
 15. A heat treatment method of a magnesium alloy, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less; (exclusive of 0 wt %) at least one rare earth element in the content range of 1.5 to 5.0 wt %; the remainder being magnesium; and unavoidable impurities, comprising the steps of: a) heating the magnesium alloy in the temperature range of 430 to 470° C.: b) quenching the heated aloloy; and c) heating the alloy to in the temperature range of 150 to 250° C. to age the same.
 16. A heat treatment method of a magnesium alloy according to claim 15, wherein the heating step a) at 430° C. is carried out 5 to 24 hours.
 17. A heat treatment method of a magnesium alloy according to claim 15, wherein the heating step c) carried out for 0.5 to 24 hours.
 18. A heat treatment method of a magnesium alloy according to claim 15, wherein the temperature range of the step c) is 150 to 200° C.
 19. A heat treatment method of a magnesium alloy according to claim 15, wherein the temperature range of the step c) is 180 to 200° C. within 0.5 to 2 hours.
 20. A heat treatment method of a magnesium alloy according to claim 15, wherein the rare earth element is a lonthanoid. 